This invention relates to apparatus for controlling the operation of a grinding system and, more particularly, to apparatus for controlling the operating condition of a grinding system containing a plurality of mills operating in a parallel fashion so as to optimize the condition of the grinding system.
A constant feed method based on a constant feed weigher (CFW) has been widely employed for feeding materials, such as minerals, to one of the mills, for example, ball mills, in a pulverizing or grinding system. Recently, a variable control method has superseded the constant feed method, however. In the variable control method, a CFW flow rate is variably controlled so as to make constant a physical quantity such as a sound pressure of the mill under control or a BE current (a drive current) for a bucket elevator for transporting the material. To effect this control, the physical quantity is collected when the grinding system produces a maximum amount of the pulverized material. The physical quantity is used as a set value for operating the grinding system under the optimum operating condition. Many disturbances frequently cause the set value to shift from what has already been set. Examples of those disturbances are change of the nature of the material, aging of balls in the mill, and others. Those disturbances are generalized as a disturbance of the first kind, which will be referred to later. This type of disturbance makes it impossible to constantly place the grinding system under an optimum operating condition.
To cope with this, Japanese Patent Disclosure (KOKAI) No. 57-194054 proposes a unique sound pressure control method which successfully removes the adverse effects caused at the start and stop of the adjacent mills in the grinding system. Another Japanese Patent Disclosure (KOKAI) No. 58-159855 proposes an approach in which a physical quantity such as the BE current and the sound pressure are variable, not fixed. The grinding system is so arranged that the disturbances, which directly influence the physical quantity, are constantly checked and the optimum quantity is automatically found. For prior art discussion, the Japanese Patent Disclosure (KOKAI) 58-159855 will be given in reference to FIG. 1. Sound generated by a ball mill 1 is sensed by microphones 8 set at toward the first and second compartment of the ball mill 1. The output signals containing the information of the sensed sounds, which are produced by the microphones 8, are passed respectively through amplifiers 9 and inverters 10 to an operating unit OP. Clinker, as the pulverized material discharged from the ball mill 1, is transported to a separator 3 by a bucket elevator 2. The clinker is classified by the separator 3 according to particle size. Part of the classified clinker is discharged and used as the product. The remainder is again put into the ball mill 1, by way of a return path 4. Before being put into the ball mill 1, the amount of the clinker, as the pulverized material, is automatically controlled by a controller CONT. Power consumed by the bucket elevator 2 is measured by a wattmeter 7 attached in the motor section (not shown). The measured quantity is then applied to the operating unit OP. A flow rate of the remainder of the clinker, which is returned to the ball mill 1 via the return path 4, is measured by an impact line flow meter 5 and is led to the operating unit OP. The operating unit OP multiplies the detected signals by predetermined coefficients, adds the products of the multiplications, and feeds the result of the addition as a process signal into the grinding system to the controller CONT. A setter, mounted in the controller CONT, contains a set value, or an optimum set value, as set therein to cause the grinding system to produce a maximum amount of the pulverized material. If no disturbance occurs in the grinding system, there is no need for altering the optimum set value. However, the grinding system essentially suffers from many types of disturbances as given below.
(1) Hardness and particle size of the clinker are not invariable. When the clinker is increased in hardness or particle size, the pulverizing rate in the ball mill 1 is reduced. As a result, the clinker discharged into the bucket elevator 2 and transported into the separator 3 contains a relatively large amount of coarse particles, and accordingly the remainder of the clinker increases. Under this condition, if an amount of the clinker fed through the belt scale 6 remains unchanged, the amount of the material under pulverization increases.
(2) Spraying water into the ball mill 1 is often required. In this case, the sound generated in the ball mill 1 changes and the pulverizing rate also changes.
(3) Temperature of the clinker changes. This causes the pulverizing rate to change.
(4) Steel balls in the ball mill 1 are worn. This results in a change of the pulverizing rate.
Many other disturbances are involved in the grinding system. Any type of disturbance, if it occurs, changes an amount of the pulverized material in the ball mill 1, viz., a load of the ball mill 1. Accordingly, an optimum set value for the grinding system is displaced from that already set in the controller CONT.
This follows from a relationship between the load and the pulverizing efficiency of the ball mill 1, as shown in FIG. 2. As seen from FIG. 2, in a range A.sub.1, the pulverizing efficiency of the ball mill 1 increases linearly with the increase of the load. In other words, the amount of the product of the ball mill 1 can be improved by increasing an amount of the pulverized material returned and fed back to the ball mill 1 under the control of the belt scale 6. The increase in the amount of this pulverized material is achieved by increasing the output signal, or the set value, of the controller CONT.
In a range B.sub.1, the relationship exhibits a reverse tendency to that in the range A.sub.1. To increase the pulverizing efficiency, it is necessary to decrease the load of the ball mill 1.
In the graph, C.sub.1 represents a point to provide the maximum pulverizing efficiency. A set value for the controller CONT is an optimum set value. The point C.sub.1 varies with various conditions in the mill system.
Manual operation, which is time consuming and troublesome, is required to find an optimum set value. Further, for the disturbances spontaneously occurring during an automatic running of the mill system, an operator must constantly supervise the operating state of the system. When such disturbances occur, the operator must again search an optimum set value for the controller CONT at that time.
In the grinding system shown in FIG. 1, to find an optimum set value, a set value for the controller CONT is automatically increased at fixed time intervals. The output signal derived from the controller CONT is integrated over fixed time periods before and after the set value is changed. If the integrated value after the set value change is larger than that before the set value change, the set value is further increased. This value is subjected to a similar comparison to the one just mentioned. Successively, this process is repeated. Then, the grinding system will find a point where the integrated value after the set value change is smaller than that before the set value change. This point is the point C.sub.1 on the pulverizing efficiency curve of FIG. 2. Thus, the grinding system confirms that the set value, as set immediately before the final change of the set value, is an optimum set value in the pulverizing system at that time, and sets the controller CONT at that value. In this way, the optimum set value is set in an automatic manner.
The process to find an optimum set value will be described referring to a flow chart shown in FIG. 3.
In a control system, in the grinding system shown in FIG. 1, upon receiving a start signal for automatic correction, the output signal of the controller CONT is integrated over a predetermined period of time. The integrated value A.sub.2 is stored in a proper memory. Then, the set value is automatically increased. When the control system in the grinding system, set at the new set value, settles down in operation, the output signal of the controller CONT is again integrated over a predetermined time period. The integrated value B.sub.2 is stored in the memory. Those integrated values A.sub.2 and B.sub.2 are compared with each other. If the comparison result is B.sub.2 .gtoreq.A.sub.2, the operation flow returns to a start point a2 in the flow chart. Conversely, if B.sub.2 &lt;A.sub.2, after the control system in the grinding system settles down at the new set value, the output signal of the controller CONT is integrated over a predetermined time period. The result of the integration, as the value C.sub.2, is stored. Further, the values B.sub.2 and C.sub.2 are compared with each other. If C.sub.2 .gtoreq.B.sub.2, the operation flows back to a point b2. If C.sub.2 &lt;B.sub.2, it returns to the point a2. Change of the integrating time and the set value depends on the conditions of the control system in the grinding system.
In this way, the set value for the mill system is automatically corrected, so that the output of the controller CONT is always kept at the highest production.
This excellent mill control method involves the following problems, however. The mill control system is neither an effective control against a process fluctuation (the disturbance of the second kind), from which every type of control method suffers, nor against the disturbances which last for a time period shorter than a response time of the mill control system. Also, the method has the problem that value is often set in the wrong direction.
The disturbance of the first kind is defined as the disturbance which will shift an optimum set value, which provides maximum pulverizing amount of the material, in the direction of increasing or decreasing the set value, horizontally in FIG. 4. The disturbance of the second kind is defined as the disturbance which will shift, regardless of the set value, in the direction of increasing or decreasing the pulverizing amount of the material, vertically in FIG. 5. Usually, this disturbance of the second kind occurs in and is common to all the parallel operating mills in a grinding system.